I am a teen in Ontario. I am starting a lawn care business. Do I have to charge HST and pay the gov?

Ask the Canadian government before you do not do it and find out you required the number to be able to tax someone.
To claim the tax back you require the entire business to be allowed by your province. They also have the information.
Under the table transactions do happen and the government simply waits for the inevitable screw up.
Operating without a business license could be federal offence. Jail time is possible.